Surface coatings are used to enhance the physical, mechanical and chemical surface properties of bulk materials, in order to reduce costs, conserve materials, and increase design flexibility. For example, optical coatings are used for lenses and mirrors and metallic coatings for electrical contacts. Coatings may be used to increase hardness and decrease wear. Protective coatings are applied to increase corrosion and oxidation resistance. Optimum coatings have excellent adhesion to the substrate to which they are applied, a dense and pore-free structure, good uniformity, and a smooth finish.
Coating techniques may be divided into two general classes, i.e. electroplating and vacuum coating. In electroplating, copper, hard and decorative chromium, nickel, cadmium, zinc, tin, silver, gold, or other materials are electro-chemically deposited onto a surface. In vacuum coating, different film production processes, such as chemical vapor deposition or physical vapor deposition, sputter deposition, ion plating, ion beam assisted deposition, arc deposition or others are used to create films.
The major difference between the two classes of coatings is the composition and/or thickness of the film and the resulting waste product production. Electroplating usually produces films from either single chemical elements, such as a metal, or a metal alloy, e.g., of two or more metals. In contrast, the majority of practical vacuum coating applications are directed toward creation of new surface chemical compounds.
The electroplating process creates a polluting waste water solution containing a high proportion of the deposited material, e.g., metal. Environmental regulatory standards are very strict and removing metals from the waste water is expensive, and requires multi-step processes. Water treatment greatly increases the cost of electro-deposition. In spite of steps taken to clean the water, only approximately 50% of the treated water is recycled in the electroplating process. The remaining water is toxic, and is released to the environment.
Ion implantation technologies are also used in vacuum coating processes. The largest disadvantage of ion implantation for industrial applications is the high cost of capital equipment and the total cost to implement the process. For example, in 1988, commercial rates for nitrogen ion implantation surface treatments were about $650.00 per batch. A batch typically consists of items mounted on a 250 cm.sup.2 rotatable plate which can be completely rotated to ensure uniform implantation. For implantation of irregularly shaped items, where specialized jigging and/or rotations are required, costs were higher, at least for the initial batch. For a cost of capital equipment of $1,000,000 and using ten years of amortizing, hourly rates of $144.12, $95.00 and $73.70 apply for one shift, two shifts, and three shifts, respectively. Accordingly, the cost of this type of vacuum coating can be significantly more expensive than electroplating.
Electroplated coatings are typically used under mild conditions, such as for decorative chrome finishes on appliances, or as corrosion resistant finishes, for example zinc, on roofing materials. If a material faces harsh or destructive conditions, vacuum coating processes are commonly used.
Electroplating provides strong adhesion between the deposited material and the substrate. In vacuum coating, it is believed that an intermix layer between the substrate and film can be used to improve adhesion. However, the majority of modern deposition techniques, such as ion beam deposition, molecular beam epitaxy, sputtering, laser ablation, or anodic and cathodic arc plasma deposition, do not produce an intermixed layer and are not appropriate for use when strong adhesion is necessary.
Several approaches have been proposed to simplify and reduce the cost of implantation processes. The proposals typically involve reduction of capital equipment cost. Simplified systems include plasma immersion, ion implantation or plasma source ion implantation, producing heavy ion beams from a solid using a metal vapor arc discharge in a pulsed mode, and metallic ion production into a PIG ion source. In the future, it may be possible to reduce the implantation cost by increasing the size of certain equipment. If the cost of ion implantation can be sufficiently reduced, it may be possible to replace electroplating with vacuum coating in many if not all coating applications.